1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device and a differential small-amplitude data transmission device, and in particular, to a differential output driving circuit which has means for adjusting the slew rate of an output signal and which is applied to, for example, an output driving circuit in conformity to a USB standard.
2. Description of the Related Art
If a step voltage is applied to a data output circuit as an input signal, the speed at which an output voltage increases is expressed as ΔV/Δt where ΔV is a change in voltage and Δt is a very short time. The ΔV/Δt is generally called an output slew rate. If the output slew rate of a data output circuit built in a semiconductor integrated circuit device (LSI) must be adjusted to regulate an output waveform from the circuit, a slew rate control amplifier or a slew rate adjusting circuit is provided. Jpn. Pat. Appln. KOKAI Publication No. 7-58559 discloses an example of a slew rate control amplifier having a function to adjust the output slew rate. Further, Jpn. Pat. Appln. KOKAI Publication No. 2002-158577 discloses an example of a slew rate adjusting circuit.
On the other hand, standards such as USB (Universal Serial Bus) 2.0 and IEEE 1394 specify the value of the output slew rate as a part of the specification of a small amplitude data signal inputted as a differential signal. For example, the USB 2.0 standard specifies the value of the output slew rate as 500 pS. Accordingly, in designing a differential output driving circuit that outputs a differential signal, it is important to eliminate the impact of the degradation of an eye pattern waveform as much as possible all over transfer lines, the degradation possibly resulting from an impedance mismatch, a parasitic concentrated capacitance, or an external noise.
In the prior art, to adjust the above described output slew rate of a differential signal in accordance with the specification, the waveform of a driving signal is controlled which is inputted to a gate of a P channel MOS transistor located on a current discharge output side of a differential output driving circuit. However, a long design period is required to obtain a desired signal waveform, thus increasing a design cost. Consequently, it is difficult to realize desired LSI products in a short period.
As described above, the conventional technique to adjust the output slew rate of a differential output driving circuit built in an LSI requires a high design cost and does not allow desired LSI products to be easily manufactured in a short period. It has been desired that these problems be solved.